Rotary machines employing rotor shafts supported by a bearing from a support structure are commonly used in gas turbine engines. During operation of these engines, the range of rotational speed of the rotor may vary from low speeds to high speeds of over ten thousand revolutions per minute.
As pointed out in U.S. Pat. No. 4,669,893 issued to Chalaire et al entitled "Annular Oil Damper Arrangement", small imbalances associated with the rotating shaft may cause the actual axis of the rotor shaft to cyclically orbit the ideal axis of the rotor shaft with a frequency equal to the angular speed of the shaft.
Such orbiting motion is commonly referred to as syncronous vibration or whirl of the shaft. The lateral or transverse vibration of the center line of the shaft away from the ideal center of rotation increases as the rotor speed of the shaft approaches the resonant frequency of the support system (critical speed) associated with one of the different modes of vibration of the shaft. The resonant frequency is a function of the stiffness of the support system.
Vibratory forces in the rotor shaft are transmitted from the rotor shaft through the support system to the engine and from the engine to its mounting structure. Because the displacement of the shaft from the ideal axis of rotation is believed to increase these forces, devices are often employed to decrease the actual displacement of the shaft from the ideal center line of the shaft.
As shown in U.S. Pat. No. 4,669,893 issued to Chalaire, one device for decreasing the displacement of the shaft and, therefore, the forces which are transmitted from the shaft to the engine and to its supporting structure is a viscous damper having a fluid damping film which is disposed between the rotor system and its support. As shown in Chalaire, the film is disposed between the bearing housing of the rotor and the adjacent structure. Other examples of such dampers are shown in: U.S. Pat. No. 4,337,983 issued to Hibner entitled "Viscous Damper"; U.S. Pat. No. 4,084,861 issued to Greenberg et al entitled "Thrust Bearing Damping Means"; and, U.S. Patent Re. 31,394 issued to Streifert entitled "Flexible Damped Bearing Assembly".
In several of these dampers, the support for the shaft includes a bearing and its housing. The bearing and housing are positioned by a spring which resists the movement of the shaft away from the ideal center line of the shaft. The damper through its fluid damps orbital motion of the shaft. Damping is especially critical if the shaft is rotated at a speed which is the natural resonant frequency of the support system. As mentioned earlier, the speed is commonly referred to as the critical speed of the support system and is a function of the stiffness of the support system.
U. S. Pat. No. 3,756,672 issued to Hibner et al entitled "Shaft Damping Arrangement" and U.S. Pat. No. 4,046,430 issued to Buono and Hibner et al entitled "Damped Intershaft Bearing and Stabilizer" show the use of viscous dampers of the type discussed above for damping vibrations in concentric shafts. In U.S. Pat. No. 4,046,430, one shaft of a pair of concentric shafts is the structure from which the positioning spring extends to the bearing housing. The shaft with the bearing housing bounds the viscous damper cavity. Thus, the support structure for the bearing is a rotating component of the engine. In this particular invention, the spring in parallel with the damper is used to change the resonant frequency of one of the shafts so that the critical speed of the other shaft is no longer in the operating range of the engine.
Another approach is to use both viscous and frictional damping to damp vibrations as is shown in U.S. Pat. No. 4,353,604 issued to Hibner et al entitled "Viscous/Friction Damper". As shown in this patent, the support system for a rotating shaft includes both a viscous damper and a friction damper which is in series with the viscous damper. The friction damper is activated when a large imbalance of the engine rotor takes place which exceeds loads normally handled by the viscous damper.
Viscous and frictional damping are shown in U.S. Pat. No. 4,213,661 issued to Marmol entitled "Bearing Support Structure Combining Fluid Damping and Spring Damping Apparatus". In this patent, a plurality of arcuate beams are disposed in the damping cavity to provide frictional damping to the bearing.
A viscous and frictional damper is also shown in U.S. Pat. No. 3,456,992 issued to Kulina entitled "Vibration Damping Device". In Kulina, the squeeze film damper uses piston ring seals which seal the damper cavity. The piston ring seals are used as frictional devices pressed by damping fluid pressure against the walls bounding the damping cavity to develop a frictional force. Because the frictional force is proportional to pressure in the cavity and because the pressure in the cavity affects the viscous and stiffness characteristics of the damping medium, it is possible to tune the damper so as to get maximum viscous and frictional damping of motion at the critical speed characteristic of the rotor shaft. This provides damping of vibrations at the critical speeds for the shaft.
The above art notwithstanding, scientists and engineers working under the direction of Applicant's assignee are seeking to develop other devices for effectively reducing vibrations from rotor whirl.